Amplifiers provide voltage gain and sufficient current and power to drive speakers. Amplifier current and power output depend on amplifier voltage output and load impedance.
dB voltage gain = 20 log (V output / V input) Current I = V / Z Power P = IV = V2 / Z
Speaker impedance is complex resistance and reactance that varies with frequency. Nominal speaker impedance is a simplification. Each point on a speaker impedance magnitude and phase graph is one frequency. Music is a combination of frequencies that changes with time. Music voltage is distributed across the audio spectrum. A speaker impedance magnitude and phase graph is a simplification.
Amplifier power is rated driving resistors with sine waves. Sine waves are one frequency. Speaker impedance varies with frequency. Music is a combination of frequencies that changes with time. Amplifier rated power is a simplification.
Passive crossover low and high pass filter impedance divides low and high frequency current and power between low and high frequency circuits.
Impedance 1 / Z = 1 / Z low + 1 / Z high
Current I = V / Z = V / Z low + V / Z high = I low + I high
Power P = V2 / Z = V2 / Z low + V2 / Z high = P low + P high
Passive Biamplification
Passive biamplification divides low and high frequency speaker circuit impedance, current, and power between separate amplifiers. The voltage output of both amplifiers is full range. The impedance, current, and power of low and high frequency speaker circuits is the same driven by one amplifier or separate amplifiers. Separate amplifiers can provide more current and power to drive low impedance.
Passive biamplification, one amplifier driving only the low frequency speaker circuit:
Current V / Z low = I low I low < I low + I high more current is available for the low frequency speaker circuit
Power V2 / Z low = P low P low < P low + P high more power is available for the low frequency speaker circuit
Passive biamplification, one amplifier driving only the high frequency speaker circuit:
Current V / Z high = I high I high < I low + I high more current is available for the high frequency speaker circuit
Power V2 / Z high = P high P high < P low + P high more power is available for the high frequency speaker circuit
One amplifier drives both low and high frequency speaker circuits with the same voltage. Separate amplifiers must drive low and high frequency speaker circuits with equal voltage to preserve speaker system frequency balance. Separate amplifiers with equal voltage input must provide equal voltage gain to provide equal voltage output.
Passive biamplification voltage output is limited by the amplifier with lower rated power. Separate amplifiers should have equal rated power.
Amplifier current and power are inversely proportional to impedance. As impedance decreases amplifier current and power increases. Most amplifiers cannot provide double the 8 ohm rated power into half the impedance, 4 ohms.
Low speaker impedance may require more current and power than one amplifier driving both low and high frequency speaker circuits can provide, clipping the peak voltage output. High frequency distortion is filtered by the passive low pass filter and passed by the passive high pass filter. Dividing low and high frequency speaker impedance, current, and power between separate amplifiers may provide more current and power to drive low impedance to avoid clipping. Distortion in the separate amplifier driving the low frequency speaker does not affect the high frequency speaker.
Dividing low and high frequency current between separate amplifiers may reduce amplifier Intermodulation Distortion IMD.
V = V low + V high
One amplifier driving both low and high frequency speaker circuits and separate amplifiers driving separate low and high frequency speaker circuits output the sum of low and high frequency voltage. Low frequency voltage and high frequency voltage cannot both equal the sum at the same time. One amplifier cannot drive both low and high frequency speaker circuits with rated power at the same time. Separate amplifiers driving separate low and high frequency speaker circuits cannot drive both low and high frequency circuits with rated power at the same time. Passive biamplification divides low and high frequency speaker circuit impedance, current, and power between separate amplifiers. The impedance, current, and power of low and high frequency speaker circuits is the same driven by one amplifier or separate amplifiers. Passive biamplification cannot provide the sum of the rated power of two amplifiers.
If one amplifier can provide sufficient current and power to drive both low and high frequency speakers without clipping, passive biamplification is unnecessary. Two amplifiers are at least twice as expensive as one amplifier. One high power amplifier may be a simpler and more cost effective solution.
Speaker damping is provided by low amplifier output impedance. Passive crossover impedance reduces amplifier damping and control of the speakers. Passive biamplification retains the passive crossover. Passive biamplification does not increase amplifier damping or control of speakers.
Multiple amplifier channels on one chassis with one power supply providing current and power for all amplifier channels such as Audio-Video Receivers may not provide enough current and power to justify passive biamplification.
A tube amplifier may be used to drive the high frequency speaker circuit with a solid-state amplifier driving the low frequency speaker circuit if the voltage gain and rated power output of amplifiers are equal. The voltage gain can be adjusted by ear using amplifier gain controls.
Passive biamplification places separate amplifiers in parallel. Separate amplifiers must have equal voltage output to preserve speaker system frequency balance. Passive biamplification may provide more current to drive low impedance compared to one amplifier driving both low and high frequency speaker circuits.
Bridging places two amplifiers in series. Bridging doubles the voltage of one amplifier, but the current limit is the same as one amplifier. Bridging rated power into 8 ohms is double the rated power of one amplifier into 4 ohms. Amplifier rated power into 4 ohms is typically less than double the amplifier power into 8 ohms because amplifier current is limited.
Passive Crossover Problems
Passive crossover impedance interacts with the speaker impedance to change the frequency response of the speaker and the intended low and high pass filter characteristics. The passive crossover must compensate for speaker impedance to produce the desired filter characteristics. Compensating for the speaker impedance adds complexity to the passive crossover.
The values of large capacitors are not precise. Electrolytic capacitors are not linear and deteriorate over time.
Long thin wire in inductors adds resistance to the speaker circuit. Iron-core inductors saturate, creating non-linear distortion. Large inductors must be spaced and oriented so they do not induce current in each other.
Resistors in the speaker circuit used to balance the levels of high frequency speaker elements waste amplifier power as heat. Resistance in inductors wastes amplifier power as heat. Resistance in the speaker circuit increases as heat increases as current increases. Resistance in the speaker circuit produces dynamic compression. Resistance in the speaker circuit interacts with speaker impedance to change frequency response. Resistance in the speaker circuit changes the frequency response of the speaker dynamically.
The passive crossover stores and releases energy, producing ringing in the speaker. Passive crossover impedance reduces amplifier damping and control of the speakers.
Active Biamplification
An active crossover divides low and high frequency voltage between separate amplifiers. Speakers are directly connected to amplifier outputs. Passive crossover impedance in the speaker circuit is eliminated. Speaker impedance does not affect active crossover characteristics. Speaker impedance does not affect frequency response. Low amplifier output impedance damps the speakers. Amplifiers control the speakers.
Sublime Acoustic K231 active electronic crossover
https://sublimeacoustic.com/products/k231-stereo-3-way-active-crossover
Siegfried Linkwitz, Linkwitz Lab, “Crossovers”, https://www.linkwitzlab.com/crossovers.htm
Rod Elliott, Elliott Sound Products, “The Benefits of Biamplification” https://sound-au.com/bi-amp.htm
https://sound-au.com/bi-amp2.htm
Rod Elliott, Elliot Sound Products, “Why Do Tweeters Blow?”, https://sound-au.com/tweeters.htm
Audiophile Heretic, July 9, 2026 Menu